1. Field of the Invention
The invention relates printing machines, and more particularly to a method for calibrating the image data to the machine condition of a digitally controllable printing machine.
2. Description of the Related Art
Such a printing machine, as disclosed, for example, by DE 295 16 830 U1, operates in particular in accordance with one of the processes of lithographic offset, gravure printing or flexographic printing. That is, a printing plate is generated once with a subject to be printed in order to produce a large number of copies of this subject from this printing plate. It is quite possible for the printing plate to be erasable and reusable (i.e., to have another subject applied to it after cleaning).
An essential feature of a digitally controllable printing machine, however, is that the image data which is to be applied to the machine are present completely in digital form, and that these are used in a manner targeted to the printing machine for setting the image on the printing plate.
Image setting is preferably carried out within the printing machine, but this is also entirely conceivable in a combination of printing-plate exposer and printing machine, which have an information link.
In order to characterize the printing system in terms of color, a so-called profiling of a printing system is carried out by a test pattern being generated from a known, machine-independent data set and printed. This test pattern normally contains fields whose structure is predefined by the individual printing inks of the output device. Thus, for example, use is made of the 1T8.7/3 color test chart for CMYK devices. Each of the measurement fields has a defined composition made up of the individual colors cyan (C), magenta (M), yellow (Y) and black (K).
Following the output of this color test chart, the color loci of the measurement fields in the color space are measured. From these measured values and the known composition of the measurement fields, the output characteristics of the printing system can be determined and a specific device profile can be drawn up. This profile indicates what color space the printer covers and how the individual color loci in the achievable color space can be reached. The coordinate system used for this is usually a device-independent color system, such as the XYZ color system, the Lab color system or the Lab (94) color system further developed from the Lab color system.
In the data preparation before printing, generally referred to as the pre-press stage, this device profile is then used to carry out a transformation from the operating color space, usually an ROB color space for scanners and monitors, into the printer color space. In this case, the profile of the operating color space is linked with that of the printer.
The conversion from one color space to another is not without its problems, since color spaces do not overlap in some areas, particular when RGB to CMYK conversions are concerned. For this reason, during the conversion additional factors may be specified, which specify the type of conversion more closely.
Overall, this procedure is generally known in the literature as color management (see, for example, EP 0 676 285 B1).
The basic idea of color management is that color originals are defined in the digital pre-press stage, irrespective of the output device and the materials used. If images defined in this way are output via a system calibrated with the effect of color management, it is theoretically, ensured that, the colorimetric appearance of the output is always identical or optimal, totally irrespective of the output process used.
If one refers to the best-known standard, xe2x80x94i.e., the ICC (International Color Consortium) standardxe2x80x94with the ICC device profiles defined therein, the device profile is always unique, under the specified illumination and measurement conditions. However, the manner of conversion into the device profile may be carried out differently. In the case of ICC profiles, for example, four different target intents are provided. The conversion may be carried out with absolute colorimetric intent, relative colorimetric intent, saturation intent and photographic intent. Absolute colorimetric intent means, for example, that the color loci are to be transformed absolutely correctly. Therefore, in theory all color values which occur in the two color spaces are identical. Those which cannot be represented in the CMYK color space must be transformed in accordance with an additional rule, for example by being placed on the boundary of the color space.
An important special case of color-space conversion is the CMYK to CMYK transformation, which converts from one printer color space into another. This is necessary, in particular, for proof purposes (the English technical term proof is to be understood as general color proof-printing methods) or in a pre-separated operating sequence, for example if CMYK data occur on the printer which do not correspond to the current profile.
The problem which is presented by a complex output device, such as a printing machine, that the profiles differ depending on the printing material used, the color used, the screen type and screen ruling used and the neutral structure used. Furthermore, the machine characteristics themselves of course go into the profile.
The parameters which do not depend on the printing machine but on the printing material, the color and the type of data preparation (screen) are referred to below as external parameters.
Even if the machine condition can be assumed to be constant, the combination of the external parameters results in a large number of different profiles.
This large number of profiles has to be kept, for example, in a database, in order then to be selected at the time of conversion. According to the prior art, this selection is made manually or by the selection of a profile in accordance with its name or its number. In addition, there is the problem that the selected profiles do not correspond exactly to the current machine condition, but to the condition in which the printing machine was when the profiling was performed. It is possible that given the same combination of external parameters, different profiles have to be applied on account of different machine conditions.
Strictly speaking, the printing machine must therefore always be put into the condition which it has inherently at the time of profiling. Because of variable environmental conditions, such as temperature and atmospheric humidity, or because of changing machine components, such as the hardness of the rubber blanket or roll imprint widths, the machine condition changes and differs from that desired.
Until now, this problem has been solved by a profile being selected and applied manually or semi-manually, but in particular not in a machine-specific manner.
In this process, use is made of the fact that the printing machine has actuating elements (inking-zone screws, ink ductor speed setting) which influence the machine condition. Therefore, to a limited extent, the machine condition can be equated to the desired condition, that is to say to the condition present at the time of profiling. However, this is costly since it entails machine time and does not produce saleable prints. On the other hand there are printing systems, for example offset printing machines printing by the anilox process which no longer have such actuating elements or no longer have them to a sufficient extent.
Although this problem exists in all printing devices, it occurs more often in printing machines having permanent printing plates, since these printing machines operate highly productively. Therefore, significant machine downtime costs occur when re-profiling has to be carried out. On the other hand, this is because readjustment, which can no longer be handled by means of the machine actuating elements, requires a new printing plate to be created, which likewise costs time and material.
The present invention is based on the object of providing a method which uses the data processing device for the final data preparation for the printing. Normally, the RIP (Raster Image Processor) automatically uses a profile which corresponds precisely with the current machine condition, that is to say can be addressed using the correct color-space conversion for the printing machine, that is to say can be calibrated.
The idea of the invention can be applied irrespective of the printing process. The invention may be implemented both in wet and dry offset, in direct or indirect gravure printing, in flexographic printing and so on.
Accordingly, the calibration of a digitally controllable printing machine having a permanent printing plate is carried out such that, at the time at which the data is prepared for image setting, a machine condition forecast for the time of printing is called up. The forecast information, together with the knowledge of the operating materials, is used to determine the corresponding printing machine profile which most closely approaches that for the print job. This profile is then used for the data preparation.
In a preferred embodiment, generic data of the printing machine or of the operating materials which relate to the color profile are additionally stored in a profile data pool. This data permits direct, data-based conclusions about the machine condition present at the time of profiling and correction to the current condition. It is therefore possible for a machine profile to include all the relevant data to allow adaptation to the machine condition of an individually addressable printing machine, and to restrict the number of necessary profiles to the extent needed.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.